Hepatocellular carcinoma (HCC) is one of the most common and deadly human cancers worldwide, and incidence and mortality for HCC is increasing rapidly in the United States. Hepatocarcinogenic mechanisms of major etiologic factors of HCC remain poorly understood. In both HCC and many pathological conditions that predispose to HCC, liver cells often highly express the inducible nitric oxide synthase (iNOS), although the role of NO in HCC is unknown. NO bioactivity is controlled not only at the level of synthesis by NO synthases, but also by enzymatic degradation, in particular by S-nitroso-glutathione reductase (GSNOR). GSNOR controls the level of protein S-nitrosylation in vivo, and is critically important for protection against iNOS-derived nitrosative stress. Human GSNOR locus is located in one of the most frequently deleted chromosomal regions in HCC. We have generated GSNOR-deficient (GSNOR-/-) mice as a genetic model to study the role of GSNOR in HCC. The overall goal of this project is to determine if GSNOR is critically important for protection against liver tumorigenesis. The specific aims are: 1) to determine whether GSNOR is critically important for protection against hepatocellular tumorigenesis in mice; 2) to elucidate the role and mechanism of GSNOR in protection of DNA repair system; 3) to test the hypothesis that GSNOR is frequently deficient in human HCC. We expect that this project will provide a novel and fundamental contribution to our understanding of the etiologic mechanism of HCC, and might lead to novel approaches for identifying patients at risk and to future preventive strategies for HCC.